The present invention relates to plastics material integral mesh structures having mesh openings defined by a generally rectangular grid of substantially parallel, orientated strands and junctions therebetween, and also to an intermediate structure which is produced when making the final structure and which has in itself commercial uses.
A problem in all integral mesh structures relates to the junctions. The junctions should be sufficiently strong without containing too much plastics material.
U.K. Patent Specification No. 982 036 describes the production of rectangular grid mesh structures by stretching a substantially monoplanar plastics material sheet comprising a pattern of holes whose centres lie on a rectangular grid; the sheet is stretched biaxially in two directions parallel to the axes of the grid. The junctions so produced are not stretched and are thick and heavy.
U.K. Patent Specification No. 1 310 474 describes structures whose junctions are strong enough for the light duty applications with which the Patent Specification is concerned but are not strong enough for heart duty applications. FIG. 5 of the Patent Specification shows two pairs of strands running into the junction, and the junction is formed of crotch filaments, a centre filament and thin webs. The webs may be about hair thickness of the strands. It is found that on rupture, the structure very often breaks at the junction, a split starting in a web, which acts as a tear starter, and running along the strands. The alternative structure illustrated in FIG. 4 is very similar except that the centre filament is absent, and its behaviour is similar.
U.K. Patent Specification No. 1 544 608 describes structures which are thin and flat, the junctions being no thicker than the strands. It is believed that a flat junction is not the strongest junction as, at the very centre of the junction, there is an area of random molecular arrangement which has less resistance to rupture than the molecularly orientated areas which surround it; the surrounding areas have the same thickness as, and are therefore stronger than, the centre area.